Help!!

So my son is 12 days old and it seems like he has an all out fight just to go poop.. I’m not talking no normal grunting, but an all out fight! Legs locked out straight, grunting, screaming and crying, for EVERY poop. I know this isn’t normal. Right now he is breastfed as well as supplemented with Enfamil Gentlease formula. Backstory: my plan was to EBF but they wouldn’t let us leave the hospital until he pooped and due to my lack of knowledge, they convinced me to give him some formula (Similac) by having me feel like he wasn’t getting enough from my breast (plus my nipples were raw and sore from him trying to get something out). Now fast forward 12 days, and i think the mix between my breast milk and the Enfamil is what’s giving him this problem. Any suggestions? He was up for hours screaming last night and when we was quiet you could hear his stomach making all kind of sounds.